The present invention relates to a circuit and a semiconductor structure for protecting integrated circuits, and more particularly, to a circuit and structure used to protect an integrated circuit (IC) from transient voltages while, for example, the IC receives a television syncrhonizing (sync) pulse of negative amplitude greater than the base-emitter turn-on voltage.
In my U.S. Pat. No. 4,484,244, which issued on Nov. 20, 1984, incorporated herein by reference, I dislcose a dual polarity electrostatic discharge protection means for an IC, the protection means being formed in an isolated region of the chip on which the IC is formed. In particular, individual protection circuits are provided for protection against positive and negative transients, respectively. Each protection circuit comprises a pair of opposite conductivity type transistors arranged as a two terminal silicon controlled rectifier (SCR). The composite structure consisting of all the circuit elements shown in said patent is formed within an "N-boat" or well, which is formed in an N.sup.- epitaxial layer on a P.sup.- substrate and is surrounded by a P.sup.+ doped region which provides isolation.
The transient protection provided by the protection means shown in U.S. Pat. No. 4,484,244 was designed to be activated when any signal exceeded the IC supply potentials. DC-coupled signals from other ICs in the system lie within the triggering thresholds imposed by the ground and supply potentials so that the protection means remains in the OFF (non-conducting) state. However, if AC-coupling is used for circuit/system reasons, the effective signal potentials can be shifted outside the ground or supply reference voltages. For example, if a composite video signal is AC-coupled to the IC, the negative-going sync tip can be several volts negative. If the protection means of U.S. Pat. No. 4,484,244 is included as part of the input circuit, the negative trigger threshold would be exceeded, turning the protection circuit ON (causing the protection means to conduct). This would cause DC restoration of the signal with the sync tip having a final potential which causes the negative transient protection circuit to conduct at approximately 0.7 volts (one V.sub.BE, the base emitter voltage) below the ground reference potential. Repetitive clamping of the signal may be undesirable for system reasons and can also cause undesired substrate current injection which could disturb the normal operation of other circuits on the same chip. In particular, forward biasing of the diode formed by the N-boat region and the P- substrate causes electron current to flow underneath the isolation region into the substrate. This current is attracted by adjacent positively biased N-boats, thereby disturbing the operation of active devices formed within the adjacent N-boats. Although the normal operation of the circuits would also be disturbed by a transient, caused for example by picture tube flashover, this is a rare event, and not synchronized and repetitive as in the case of sync pulses.
Modification of the protection means described in U.S. Pat. No. 4,484,244 to enable the negative firing threshold to be extended to several volts below (negative) ground, while maintaining full protection against undesired transients which exceed the new threshold, is described herein.